Hearing aids known from the prior art have hearing programs which differ from one another, each of which corresponds to a predefined transmission characteristic of the hearing aid, in particular a frequency-dependent one. An application example of the switching over of a predefined hearing program is the operation of the hearing aid during a telephone call. Hearing aids known from the prior art are designed to switch over to another program when a telephone handset is brought near to the hearing aid, whereby for this purpose hearing aids known from the prior art have a sensor for detecting a magnetic field. The telephone handset has a permanent magnet to create a magnetic field, to switch over to another program. Hearing aids known from the prior art can have, for example, a REED contact or a GMR sensor (GMR=giant magneto-resistive) for detecting a magnetic field to switch over a hearing program.
Such small magnets, which are however strong, produce an unpleasant whistling if they are incorrectly positioned in the neighborhood of the hearing aid, due to feedback in an inductive coil used to detect a telephone signal, a so-called telecoil. Another problem can arise due to the fact that other external magnetic fields can cause an unintended switchover to another hearing program. As a further disadvantage, a sound generator can be irreversibly damaged by an external magnetic field.